This project is concerned with cellular regulatory factors affecting DNA replication, particularly those pertaining to the enzymology and control of DNA precursor synthesis. The principal experimental material is the technically advantageous bacterium, Escherichia coli, and its bacteriophages, particularly T4. Elements of the proposed research include the following: (1) molecular characterization of phage-coded enzymes which apparently play multiple roles in the phage-infected cell, including dihydrofolate reductase, deoxycytidylate hydroxymethylase, and deoxynucleotide kinase; (2) analysis of the defective functions associated with phage mutants bearing hitherto uncharacterized defects in DNA replication; (3) investigations of the kinetics of DNA precursor metabolism; and (4) exploration of the mechanism by which bacterial cells recognize hydroxymethylcytosine-containing DNA and degrade it when the hydroxymethyl groups on the DNA are not protected by glucosylation.